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WildBill wrote:

Spluloacle wrote:F stop BTW is the number that tells you your focal point (i think) So if you shoot an image with an f 1.8 you have a small amount of picture in focus and the rest is blurred. A pic shot with an f 3.5 is pretty sharp with a nice background blur. GENERALLY.

ISO basically (how I understand it anyway) is a low number lets in less light and a high ISO lets in more.

Not quite. The f-number is the focal length divided by the "effective" aperture diameter of the lens. There are fixed f-number lens which are generally more expensive than variable f-number lenses. Unless you spend quite a bit of money, most zoom lens will have variable f-numbers.

ISO relates to the speed [sensitivity] of the film. An ISO 400 film is twice as sensitive to light as an ISO 200 film. That means that you can use a higher ISO film in lower light situations and still get proper exposures. Since a DSLR doesn't use film, the ISO is a setting for the relative sensitivity of the light sensor. Whether or not the ISO setting on a DSLR relates directly to film sensitivity, I am not sure.

In order to shoot a properly exposed photo, there has to be a certain amount of light that is focused on the film. Shooting pictures with ambient lighting [with no flash] there are two ways to change the amount of light reaching the film - change the shutter speed or the F stop. If you adjust your lens to a lower F stop it opens the aperature and lets in more light - a change in one f stop doubles the amount of light. Shooting with a larger aperature [lower F stop] descreases the depth of field - so the subject will be in sharp focus and the background will be blurred. The F stop is a little confusing. Changing from F5.6 to F4.0 opens the lens by one F stop, which will double the amount of light reaching the film. Comparing two lenses, such as an 2.0F and a 2.8F, you would say the 2.0F is a "faster" lens since it is able to let in twice the amount of light with a fully opened aperature.

If you change the shutter speed you change the amount of light exposing the film. Increasing the shutter speed from 1/125 second to 1/60 second lets in double the amount of light. If you are shooting a moving object, the photo shot at the higher speed will be less blurred.

Choosing the proper combinations of shutter speed and F-stop is one of the many technical choices that a photographer can make with an SLR.

I hope that helps a little.